High-frequency power measuring apparatus



Sept. 22, 1953 E. GlNz-roN 2,653,299

HIGH-FREQUENCY POWER MEASURING APPARATUS Original Filed Feb. 4, 1942INVENTOR 50W/:R0 LG/NZTON ATTORNEY Patented Sept. 22, 1953HIGH-FREQUENCY POWER MEASURING APPARATUS Edward L. Ginzton, Los Altos,Calif., assignor to The Sperry Corporation, a. corporation oi DelavareOriginal application February 4, 1942, Serial No.

1950, Serial No. 170,574

5 Claims. I

This invention relates generally to devices for measuring high frequencypower and more speciiically to apparatus for use in the absolutemeasurement of power in ultra high frequency electromagnetic energytransmission systems.

This application is a division of co-pending application Serial No.429,508 filed February 4, 1942 and now abandoned.

As taught by the aforementioned parent application, a Wheatstone bridgemay be arranged With a barretter element and three additional bridgeresistor elements, connected and arranged to be supplied with directcurrent power or low frequency power in such a way as to provideappreciable dissiptation in the bolorneter or barretter element and anattendant rise or predetermined change in resistance thereof. Microwavepower may then be supplied to the barretter element of the bridgetending to cause a further rise in its temperature, and correspondingfurther increase in its resistance. By reducing the direct current poweror low frequency power to the extent to restore the bridge to a balancedcondition (i. e. to return the barretter element to the temperatureinitially due to D. C. power alone),

and by making direct-current measurements to ascertain the extent of thedecrease in the direct current power supplied to the barretter unit. theuser is enabled to obtain a basically accurate measure of the microwavepower supplied to the l barretter element.

The present invention has to do with the special feature of arrangingone of the bridge resistance elements to be connected adjacent to thebarretter element as a temperature com- 5;

pensating resistor. this resistor comprising a coil of wire havingappreciable resistance and having a temperature coelcient of resistivityso related to the temperature coefcient of the barretter element as tohave the bridge remain balanced i.

ambient temperature before the compensating l resistor iscorrespondingly affected.

It is another object to achieve a construction of the inventioncombining efficiency and eiiectiveness with simplicity of construction.

These objects are met in the present invention Divided and thisapplication June 27,

by arrangement of the compensating resistor as a coil wound around theouter conductor of a coaxial transmission line, the bolonietric orbarretter element being included as a portion of the inner conductor ofthe coaxial transmission line and being surrounded by the compensatingresistor winding or a substantial portion thereof.

An embodiment of this invention will now be described with reference tothe drawings, wherein Fig. 1 is a circuit diagram of a barretter bridgefor measurement of ultra high frequency or microwave power, and

Fig. 2 is a drawing in cross section of the coaxial linc structureincluding the barretter unit and the compensating winding coaxialtherewith and external of the transmission line.

Referring now principally to Fig. l, the Wheatstone bridge 8 is showncomprising resistors li and I2 and temperature sensitive resistors I and14. Resistor 14 is a barretter unit, and resistor l0 is a compensatingresistance coil, to be described more fully hereafter. A battery I isconnected to terminal A of the bridge 8, and is arranged to supplydirect current power to the bridge through resistors IS, lla. la andlEa, the last-named resistor being connected to bridge terminal B. Theseresistors are provided for the purpose of enabling a user of the bridgeto control the direct current energizaton thereof, over a relativelywide range of adjustments. A series of resistors Ilb, i619, I5b, I4 andI3 are connected also between resistor I8 and bridge terminal A toprovide a resistance path of substantially equal value to the paththrough the bridge 3. Resistors I8, Hb, lb and |51) are arranged fordecrease of resistance when the resistances of the elements in thebridge branch circuit are increased, and for increase when the latterare decreased, for keeping a substantially constant current load onbattery l during the use of the bridge system.

The galvanometer 20 is connected with a double-throw switch 2| incircuit with the bridge 8 and arranged so that the galvanometer may beconnected directly between bridge terminals C and D for indicating tothe user a condition oit' balance by neutral reading of the pointer ofthe D. C. galvanometer, and for indicating the direction and extent ofdepartures from the balanced condition in bridge 8.

A direct current voltmeter 2 and a calibrated decade voltage dividersystem 3, 4, 5, 6, are connected across the current supply battery I forcontrolling the potential of terminal 1, the potential of this terminalbeing accurately known at all times by reference to voltmeter 2 and thereadings of the decade resistor system 3, 4, 5. One terminal 22 of thedoublethrow switch 2| is connected to terminal 1 in such a way that thegalvanometer 20 may be transferred from its direct connection across theterminals C-D of bridge B to a circuit connection enabling the user toascertain when he has achieved equal poten.- tials at terminal 1 andterminal C of bridge 3- i. e., a potential equal to the potential ofterminal D relative to terminal A when the bridge is balanced.

Fig. 2 illustrates the physical arrangement of the barretter 14 and thecompensating resist ance coil l0, together with suitable structure foreiiiciently supplying microwave power to the barretter 14. Barretter 14is illustrated as comprising the fine wire element of a unitcommercially known as the Littelfuse. The structure for receiving theLittelfuse cartridge 15 comprises a coaxial transmission line havingtubular outer conductor sections 8l and 84 joined together at 82 with athreaded section and a clamping nut 83 for ease of disassembly andreplacement of the barretter unit 14, 15.

A high frequency by-pass capacitor is provided in the left-hand end ofthis coaxial transmission line. This capacitor comprises a fixed piston19 and a thin layer of solid dielectric material 8l), providing for thesupport of the piston or block 19. One fuse and receptacle cup 15 isattached to the block 19. The other receptacle cup 11 is formed at theleft-hand end of the coaxial line inner conductor 1S, which,

together with the barretter unit, comprises the f current connection tothe inner conductor 18. Y

A cylindrical body 90 which may be constructed of highly conductivemetal is arranged on inner conductor 18 to be located at a positionwithin conductor 84 for achievingr impedance transformation. This block,preferably one-fourth Wavelength long, is adjusted to the position alongtransmission line 18, 34 at which it accomplishes an impedance inversionfrom the value at its left-hand end due to the barretter and capacitorand the stub 9d, 9|, into the desired resultant impedance at itsrighthand end.

The compensating resistor l0 is wound around the tubular outer conductorsection 8| in such a way that at least a substantial portion of thiswinding surrounds the barretter. The ends of this insulated resistorwinding Ill are connected to suitable connector pins in a multicircuitterminal unit 88 which is provided in the end of a protective shell 86,81, attached to the outer conductor of the transmission line andarranged to shield and protect the compensating winding Il). Aconnection is also provided between one pin or element of themulticircuit terminal unit 8B and piston or block 19. These terminal connection elements are thus adapted to be connected directly in circuitwith the other elements of the bridge 8 (Fig. l) together With aconnection to the terminal formed at the outer end of conductor 90.

The compensating resistor I0 preferably has Ill) substantially equalresistance to the resistance of the barretter Wire 14, and also has asubstantially equal magnitude as well as similar sign of its temperaturecoeiiicient of resistivity. Its length and cross-sectional area are ofcourse of greater order of magnitude than the corresponding dimensionsof the barretter element.

In operation of the bridge system, the supply switch in series withbattery I is closed, and the galvanometer 20 is connected across thebridge circuit by transfer of the switch 2l to contact with terminal 50.The arm i9 of potentiometer i8 is set substantially to its midpoint, andthe ganged adjustable control elements of the re sistor portions I1A and11B are set for equal resistance values therein. Resistors IEA and IBBand IBA and 16B are adjusted to relativelylow resistance values toprovide appreciable direct current energization of bridge 8, andappreciable resultant direct current heading of the barretter 14.

Resistor H is adjusted to bring about a condition of balance in thebridge, as indicated by a neutral pointer setting in galvanometer 20.

The switch 2l is now transferred to its opposite throw. and the decaderesistor system 3, 4, 5 is adjusted to bring about a neutral indicationof the galvanometer 20, and thus to eiect the condition that terminal 1is at the same potential relative to terminal A as is bridge terminal C,and hence, at the same potential as bridge terminal D. This potentialvalue is obtained from the voltage divider settings of the decadevoltage divider resistor system 3, 4, 5, 6, taken with reference to thebase voltage indicated by voltmeter 2. From this potential value of thedirect voltage equal to the potential drop across the barretter 14, andthe resistance value of this barretter as indicated by the resistance ofresistor Il, the initial direct current power being dissipated in thebarretter 14 is obtained by Ohms law as Now, the high frequency power isintroduced in the apparatus of Fig. 2 through the righthand end 92thereof, further increasing the temperature of barretter 14, and causingfurther change of its resistance, bringing about an unbalanced conditionin bridge 8. .Switch 2| is now transferred to connect the galvanometer20 across the bridge C, D; arm Il of the bridge is retained aspositioned before the introduction of the high frequency power, andresistors 11a, I1b and I8 are readjusted for increased resist ance inthe current branch through the bridge and correspondingly decreasedresistance of the compensating branch to restore balance in the bridge.Thus, the direct current energization of the bridge B is reduced, whilethe load on batn tery I is retained substantially constant. In this way,the user is assured that the direct current power supplied to thebarretter 14 has been reduced to an extent exactly equal to the addedincrement of high frequency power supplied to the barretter anddissipated therein.

Now, switch 2l is again transferred to connect galvanometer 2d betweenbridge terminal C and switch terminal 22, and voltage divider system 3,4, 5 is again adjusted to obtain a neutral indication in galvanometer29. The reduced direct potential reading of the voltage divider system3, 4, 5 is now taken, and the square of this voltage divided by theresistance value of the barretter 'I4 yields the measure of the reduceddirect current power supplied to the barretter 14. The differencebetween this reduced direct-current power value and the initialdirect-current power value gives the precise extent of thedirect-current power reduction, and hence the magnitude of the highfrequency power being dissipated by the barretter.

With ambient temperature changes, the operation of the bridge system issubstantially unaffected, since ambient effects are equally applicableto the compensating resistor I0 and the barretter wire 14.

This system of measurement provides the advantages of very highaccuracy, and of freedom from dependence upon precalibration by acalorimeter or other high frequency basic measuring device, and alsoprovides for a convenient unit with the barretter for receiving the highfrequency power, the system being fully protected against errors arisingfrom ambient temperature changes.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A compensated bridge for use with a high frequency transmission linecomprising a pair of standard resistors, a third resistor comprising aWire within an envelope connected in series with the transmission line,said third resistor having a resistance varying in accordance withchanges in the high frequency energy, and a temperature compensatingresistor, the temperature compensating resistor being in the form of awire wound around the envelope of the thirdmentioned resistor andexteriorly of the transmission line, the standard resistors, said thirdresistor, and said compensating resistor being connected to form abridge circuit.

2. In a high frequency wattmeter, a concentric transmission linecomprising inner and outer conductors for transmitting saidultra-highfrequency energy, a barretter wire unit disposed in serieswith the inner conductor, a temperature compensating resistor woundaround the outer conductor of said concentric transmission line, and abridge circuit including said barretter wire unit and said temperaturecompensating resistor as arms of the bridge, the balance of the bridgebeing aiected by changes in resistance of the barretter wire unit withchanges in the high frequency energy transmitted by the transmissionline.

3. Apparatus for measuring high frequency energy, said apparatuscomprising a section of coaxial line adapted to transmit said highfrequency energy, the coaxial line having an inner conductor and anouter conductor, a barretter wire unit disposed in alignment with saidinner conductor and connected in series therewith, a

temperature compensating resistor in the form of a resistance wire coilconcentrically positioned around the coaxial line section and saidbarretter wire unit, a direct current source, said barretter Wire unitand compensating resistor being r'onnected in series across said directcurrent source, a pair of standard resistors connected in series acrosssaid direct current source, and a galvanometer connected between thejunction of said standard resistors and the junction of the barretterwire unit and the temperature compensating resistor.

4. A high frequency power sensitive apparatus suitable for use in abridge wattmeter circuit, said apparatus comprising a coaxial linesection having an inner conductor and a tubular outer conductor,capacitor means connected between said inner and outer conductorsadjacent one end of the coaxial line section, said capacitor meansproviding an effective short circuit to high frequency current and aneffective open circuit to direct and low frequency currents, aresistance element connected in series with the inner conductor, a coilof resistance wire wound around the other portion of the outer conductorand surrounding the resistance element, the resistance and temperaturecoefficient of resistivity of the coil being substantially equal to theresistance and temperature coefficient of resistivity of the resistanceelement, shielding means enclosing the coil, and electrical conductormeans for providing external connections between the bridge wattmetercircuit and the coil and resistance element respectively, the length andcross-sectional area of the resistance wire in the coil being many timesgreater than the length and cross-sectional area, respectively, of saidresistance element, whereby the resistances of the coil and resistanceelement change equally with ambient temperature changes but only theresistance of the resistance element is affected materially by changesin current therethrough.

5. Apparatus as dened in claim 4 wherein said electrical conductor meansincludes a quarter wavelength coaxial stub having an inner and outerconductor, the inner conductor being connected to the inner conductor ofsaid coaxial transmission line section adjacent the resistance elementfor providing an external direct current connection to the resistanceelement.

EDWARD L. GINZTON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,673,681 Hull et al. June 12, 1928 2,127,408 Kaar Aug. 16,1938 2,270,949 Hulster Jan. 27, 1942 2,274,346 Roosenstein Feb. 24, 19422,344,298 Green Mar. 14, 1944 2,405,174 Aford Aug. 6, 1946 2,435,442Gurewitsch Feb. 3, 1948 2,464,277 Webber Mar. 15, 1949 2,473,495 WebberJune 14, 1949 2,516,944 Barnett Aug. l, 1950

